Magnesium-base alloy



` March 6, 1962 G. s. FoERsTER ETAL 3,024,107

MAGNESIUM-BASE ALLOY Filed Jan. 20, 1960 Geo/ye 6. ,Koers/er Y L/Ogdf. Raaf/da gym e.

United States Patent O 3,024,107 MAGNESIUM-BASE ALLOY George S. Foerster, Midland, Mich., and Lloyd E. Rautiola, Fords, NJ., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Jan. 20, 1960, Ser. No. 3,685 11 Claims. (Cl. 75-168) The invention relates to magnesium-base alloys. It more particularly concerns magnesium-base alloys containing both thorium and manganese and having among other desirable properties high strength not only at room temperatures but also at elevated temperatures.

This is a continuation-in-part of copending application Serial No. 663,614, filed June'S, 1957, now abancloned.

Among the magnesium-base alloys having good elevated temperature properties is the ternary alloy of magnesium, Ithorium, and manganese disclosed in U.S. Patent 2,774,664. This'alloy contains from 0.5 to 8 percent of thorium and from 0.2. to Z percent of manganese. We have now found that by including in the-alloy small amounts of zinc, that is, from 0.1 to 1.2 percent, an unexpected increase in the physical properties of the alloy is obtained and improvement persists even when the manganese content is as low as 0.1 percent. The alloy, for example, exhibits especially good strength properties even when subjected to temperatures as high as 800 or 900 F.

The invention then is based upon the discovery that by the addition of a small amount of zinc to the magnesiumbase magnesium-manganese-thorium ternary alloy, in which the so-added zinc is soluble, increased strength is obtained where other magnesium-soluble alloying metals, such as aluminum, produce a contrary effect. The alloys obtained according to the invention have a wider range of usefulness than the parent ternary alloy. In certain composition ranges, hereinafter defined, the alloy exhibits higher strength properties not only at elevated temperatures but also at ordinary ambient atmospheric temperatures. In other composition ranges the alloy exhibits excellent creep resistance. The alloys also have the advantage of being susceptible to further improvement by the addition of a rare earth metal, e.g., ccrium, praseodymium, lanthanum, neodymium, or mixtures of them, such as didymium and misch metal, as in amounts of up to 4 percent by weight with a corresponding reduction in the magnesium content of the alloy.

The single figure of the appended drawing is a bilinear graph having defined thereon closed figures representing imited ranges of weight percentages of manganese and of zinc which are employed together in formulating the present improved alloy. A given point within an area defines at once both a manganese proportion and a zinc proportion which may 'be used in making up one of the alloy compositions of the invention.

In carrying out the invention, in its broadest aspects, from 0.5 to 8 percent by weight of thorium, from 0.1 to 2 percent of manganese, and from 0.1 to 1.2 percent of zinc, the range of percent of manganese and of zinc being further limited to those manganese and zinc proportions delined by that closed figure of the drawing bounded by the lines connecting points A, B, C, D and E, are alloyed together as by melting in a steel pot in the presence of a saline ux suitable for magnesium.

More preferred ranges of proportions of the alloying metals are from 1 to 4 percent of thorium, from 0.2 to 2-.0 percent of manganese, and from 0.1 to 0.8 percent of zinc, the manganese and zinc percents falling within Patented Mar. 6, 1962 the closed area of the drawing bounded by the lines connecting points F, G, D, H and I, the balance being magnesinm except for incidental impurities which may be present in the metals used. The incidental impurities usually do not exceed 0.3 percent of the weight of the alloy.

A range of compositions of the alloy having widest utility contains from 1 to 4 percent of thorium, 0.3 to 1.5 percent of manganese, 0.2 to 0.5 percent of zinc, the manganese and zinc proportions being further limited to those proportions falling within the closed area of the drawing bounded by the lines connecting points I, K, L, M and N. As aforesaid, up to 4 percent of misch metal may also be added with a corresponding decrease in the magnesium content, about l percent being preferred.

The alloys of the invention containing from 0.2 to 0.4 percent of zinc, 0.3 to 0.9 percent of manganese and 1 to 4 percent of thorium, the balance being magnesium, exhibit especially improved creep resistance at both room temperature and elevated temperatures. A specific alloy composition containing low manganese and zinc concentrations and exhibiting good creep resistance consists of 2.5 percent of thorium, 0.25 percent of manganese, 0.35 percent of zinc, and the balance magnesium.

Alloys of the invention containing at least 0.3 percent of zinc and at least 0.7 percent of manganese exhibit improved static strength properties. In an especially preferred range of compositions exhibiting the desired strength properties the alloys contain from 0.3 to 0.5 percent of zinc, 0.7 to 1.2 percent of manganese, and from 1 to 4 percent of thorium, the balance being magnesium.

The order in which the metals are alloyed does not appear to be critical. It is generally preferable first to melt the required amount `of magnesium in the presence -of a suitable saline ilux for magnesium, such as for example one comprising by weight 34 percent of potassium chloride, from 7 to l0 percent of barium chloride, from 9 to 12 percent of calcium liuoride, from 27 to 3l percent of magnesium chloride, and from l0 to 13 percent of magnesium oxide. After melting the magnesium and heating it to between about 1350 and 1400 F., the requisite amount of manganese may be stirred into the melt followed by the thorium. Thorium is available as a hardener in the form of a binary magnesium-thorium alloy containing 28 percent of thorium. Such a hardener or thorium itself in the requisite amount to provide the desired thorium content may be used. Zinc may be added as metallic zinc or preferably as a magnesium-zinc eg., misch metal, if used, may follow or precede 4the addition of the other alloying elements. The melt is stirred for a sufficient time (e.g., 20 minutes for a 500 pound batch) to ensure dissolving the alloying metals in the magnesium.

After the metals are alloyed, the stirred melt is allowed to settle by holding it quiescent for l5 to 20 minutes or longer as desired. Prior to the settling stage of the al loying operation, some zirconium, eg., about 0.5 percent, may be added as for example in the form of a magnesiumzirconium hardener or other convenient form. This serves to improve the malleability of the alloy even though most of it settles out in the settling stage. The settling permits the impurities as well as the flux to separate from the molten alloy. The settled alloy is decanted off and cast in any convenient manner.

In the following tables illustrative examples are given of the alloy and properties obtainable.

In the examples of Tables -I vand Il, the alloys were prepared as described and the blanks similarly then cast into ingot 2 inches thick. The ingot was heated to between about 900 and 950 F. then rolled in successive passes producing from 10 to 20 percent reduction in thickness per pass'until the thickness decreased to 0.100 inch. The so-rolled metal was then annealed for one hour at 1000" F. followed by quenching in water. The so-treated metal was then cold rolled (about 180 F.) in multiple passes to a thickness of 0.085 inch followed by heat treatment at 750 F. for 1 hour. '1`he creep strength of a number of specimens of the alloy of the invention so-prepared is set forth in Table I together with the creep strength of a similarly prepared specimen of the parent alloy for comparison. .The creep strength of a specimen of an alloy having -a composition close to lbut outside the scope of the present invention is also listed for comparison as No. 1l in the table.

Table I Alloy Stress in 1bs./sq. in. re-

quired to produce 0.1% creep extension in 100 Composition hours at- Percent Percent Percent 600 F. 650 F. 700 F.

Th Mn Zn In Table II, the tensile and compression strengths are given of specimens of some of the same alloys as those of Table I together with additional examples of alloys similarly prepared.

Table Il Alloy Compres- Tensile sion yield yield Composition 1 strength in strength 2 p.s.i. X in p.s.i. X No. 1,000, room 1,000, Percent Percent Percent. temp. 800 l" T Mn Zn 1. 5 0. 3 0. 3 17 5 1. 5 0.45 0. 3 22 7 l. 5 0. 7 0.3 18 9 2. 0 0.3 0. 5 22 3 2. 0 0. 6 0. 5 2l 5 2. 0 0. 8 0. 5 21 8 2. 0 1. 1 0. 5 22 S 2. 0 1. 7 0. 5 22 4 2. 5 0. 7 0.3 26 5 16 2. 5 0. 7 21 3 9 2. 0 0.2 0. 5 21 1 3. 0 0. 6 0. 3 22 9 3. 0 0. 9 0.3 22 11 3. 0 1. 4 0.3 24 10 3. 0 1.8 0.3 24 8 1 Balance commercial magnesium 99.8% pure.

I Measured at 0.2% deviation from the modulus line. l 700 F.

In Table III room temperature properties of various specimens of the alloy are given as further examples together with blanks ofthe parent alloys for comparison. A11 these specimens were prepared by heating an ingot of the alloy to between about 900 and 950 F. and rolling the ingot hot at to 20 percent thickness reductions per pass to a thickness of 0.150 inch. The so-rolled alloys were heat treated for one hour at 1000 F. and then quenched in water. IEach quenched alloy was then reduced in thickness to 0.075 in by Ifurther rolling through steam heated rolls followed `by heat treating at 500 for 8 hours.

Table Ill Alloy Room Temperature Properties Composition Tensile Yield Per- Compres- N o. strength, strength, cent sive Yield Pcr- Per- Perp.s.i. p.s.i. E strength, cent cent cent p.s.i.

Th Mn Zn 18.. 1. 2 0. 0. 7 42,000 34, 200 4. 5 33, 400 l. 2 0. 7 42,000 35,000 3. 5 30, 000 l. 2 0. 7 0. 3 40. 600 33, 200 4. 0 31, 600 2. 0 0. 4 0. 7 44, 400 36. 800 3. 5 36, 400 2. 0 0. 7 42, 000 35. 000 3. 5 30, 000 2. 0 1. 5 0. 7 45, 600 3T. 700 5. 0 37, 600 2.0 0. 7 1. 2 46,000 3S, 000 4. 0 36,400 0. 5 0. 7 0. 7 39, 000 400 9. 0 29, 600

Nora-Yield strengths measured at 0.2% deviation from modulus line.

As aforesaid small amounts of misch metal (designated MM below) may be added advantageously for still further improvement in the elevated temperature properties of the alloy as indicated in the data of Table IV. lThe alloys used were prepared as already described and cast into ingot which was heated to between 900 and 950 F. and rolled into sheet 0.125 inch thick. The rolled sheet was annealed at 900 F. for one hour, water quenched, and then cold rolled to linal thickness. The so-prepared sheet was then heat treated at 715 F. for one hour. The tensile yield strength of specimens of the sheet was measured, at 0.2 percent deviation from the modulus line, both with and without ageing as indicated in the table.

Table IV Alloy Tensile yield strength in 1000s p.s.i. at 600 F. Composition Percent Percent Percent Percent 1 2 3 Th Mn Zn M M 2.15 0. 5 0. 3 1.0 18. 5 18.6 20. 4 2. 15 0. 5 0. 3 14. 5 16. 5 No test l. 0 0. 5 0. 3 1. 0 16. 0 18.0 19.8 1. 0 0. 5 0.3 14. 7 16.0 No test 1. 0 0. 83 0. 77 1. l 12. 2 14. 2 16. 5 1. 0 0. 83 0. 77 10. 9 13. 7 N0 test 2. 0 0. 77 0. 73 1. 1 14. 4 13. 8 19. 8

(l) 50% cold rolled, no ageing. (2) 50% cold rolled, aged at 500 F. for 8 hours. (3) 20% cold rolled, aged at 500 F. for 8 hours.

Among the advantages of the invention are that the alloy possesses the characteristic lightness of magnesium and high strength at elevated temperature without sacriiicing desirable room temperature strength and malleability.

We claim:

l. A magnesium-base alloy containing from 1 to 4 percent of thorium, 0.1 to 1.5 percent of manganese, and from 0.1 to 0.3 percent of zinc, the balance being magnesium.

2. An alloy according to claim l in which a rare earth metal in amount up to 4 percent replaces a corresponding amount of magnesium.

3. An alloy according to claim l in which misch metal in amount up to 4 percent replaces a corresponding amount of magnesium.

4. A magnesium-base alloy containing 2.5 percent of thorium, 0.7 percent of manganese, and 0.3 percent of zinc, the fbalance being magnesium. v

5. An alloy according to claim 4 in which a rare earth l ing points A, B, C, D and E to form a closed figure, the balance being magnesium.

7. An alloy according to claim 6 in which a rare earth metal in amount up to 4 percent replaces a corresponding amount of magnesium.

8. A magnesium-base alloy containing from 1 to 4 percent of thorium, 0.2 to 2 percent of manganese and from 0.1 to 0.8 percent of zinc, the proportions of manganese and of zinc being further limited to those proportions defined by that area of the drawing bounded by the lines connecting points F, G, D, H and I to form a closed iigure, the balance being magnesium. v

9. A magnesium-base alloy containing from 1 to 4 percent of thorium, 0.3 to 1.5 percent of manganese and from 0.2 .to 0.5 percent of zinc, the proportions of manganese and zinc being further limited to those proportions defined by the area of the drawing bounded by the lines connecting points J, K, L, M and N to vform a closed gure, the balance being magnesium.

10. A magnesium-base alloy containing from 1 to 4 -percent of thorium, 0.3 to 0.9 percent of manganese, from References Cited in the file of this patent UNITED STATES PATENTS 2,774,664 MacDonald Dec. 18, 1956 FOREIGN PATENTS 759,411 Great Britain Oot. 17, 1956 

1. A MAGNESIUM-BASE ALLOY CONTAINING FROM 1 TO 4 PERCENT OF THORIUM, 0.1 TO 1.5 PERCENT OF MANGANESE, AND FROM 0.1 TO 0.3 PERCENT OF ZINC, THE BALANCE BEING MAGNESIUM. 